In recent years, organic electronic devices in which organic compounds are used as semiconductor materials have enjoyed remarkable prosperity. Typical examples of their applications include organic electroluminescent devices (hereinafter referred to as organic EL device) that are expected to become next generation flat panel displays, organic thin film transistors (organic TFT) that are attracting attention on account of their ability to furnish thin film transistors to be used for driving pixels in displays by a low-cost process such as printing and to cope with flexible substrates, and photovoltaic devices (organic thin film solar cell) that provide lightweight and flexible power sources.
In the manufacture of a semiconductor device using silicon which is an inorganic semiconductor material, the forming of a thin film of silicon necessarily employs a high temperature process as well as a high vacuum process. The need of a high temperature process makes it impossible to form a thin film of silicon on a plastic substrate. Hence, it has been difficult to make a product in which a silicon-based semiconductor device is incorporated as a part flexible and lightweight. On the other hand, the need of a high vacuum process has made it difficult to enlarge the area and lower the cost of a product in which a semiconductor device is incorporated as a part.
An organic compound is easier to process than inorganic silicon and its use as a semiconductor material is expected to realize an inexpensive device. Further, a semiconductor device using an organic compound can be manufactured at low temperature and its application to a variety of substrates including plastic substrates becomes feasible. Still further, an organic semiconductor material is structurally soft and a combination of a plastic substrate and an organic semiconductor material is expected to be applied to the manufacture of organic semiconductor products in which the characteristics of the two in combination are fully utilized, for example, to the realization of flexible devices such as organic EL panels and electronic paper, liquid crystal displays, information tags, and large-area sensors such as artificial electronic skin sheets and sheet type scanners.
The organic semiconductor materials intended for use in the aforementioned organic electronic devices are in need of improvement of properties; for example, enhancement of the luminous efficiency, extension of the life, and reduction of the driving voltage in the case of organic EL devices, improvement of the charge mobility to lower the threshold voltage and improve the switching speed in the case of organic TFT devices, and enhancement of the photovoltaic conversion efficiency in the case of organic thin film solar cells.
For example, in the case of materials for organic EL devices, a host material that plays the role of charge transport in the light-emitting layer becomes important in order to enhance the luminous efficiency. Of the host materials proposed thus far, typical examples are 4,4′-bis(9-carbazolyl)biphenyl (hereinafter referred to as CBP), a carbazole compound presented in patent document 1, and 1,3-dicarbazolylbenzene (hereinafter referred to as mCP) presented in non-patent document 1. Since CBP is characterized by having a good hole transfer property but a poor electron transfer property, the use of CBP as a host material for tris(2-phenylpyridine)iridium complex (hereinafter referred to as Ir(ppy)3), a typical phosphorescent green light-emitting material, disturbs the balanced injection of charges and causes an excess of holes to flow out to the side of the electron-transporting layer. The results is a reduction in the luminous efficiency of Ir(ppy)3. On the other hand, the use of mCP as a host material for bis[2-(4,6-difluorophenyl)pyridinato-N, C2′] (picolinate) iridium complex (hereinafter referred to as FIrpic), a typical phosphorescent blue light-emitting material, displays relatively good luminous characteristics, but the compound is not satisfactory for practical use particularly from the viewpoint of durability.
As described above, host materials that are well balanced in the injection and transport characteristics of electric charges (holes and electrons) are required in order for organic EL devices to display high luminous efficiency, Furthermore, compounds that are electrochemically stable, highly resistant to heat, and excellently stable in the amorphous state are desirable and further improvements of properties are demanded.
In recent years, organic semiconductor materials that are comparable to amorphous silicon in charge transport characteristics are reported as useful for organic TFT devices. For example, non-patent document 2 presents an organic TFT device in which pentacene, an acene type polycyclic aromatic molecule formed by rectilinear fusion of five benzene rings, is used as an organic semiconductor material and reports that the device displays a charge mobility comparable to that of amorphous silicon. However, in the case where pentacene is used as an organic semiconductor material for an organic TFT device, a thin organic semiconductor film is formed from pentacene by the vapor deposition process in superhigh vacuum and this is disadvantageous from the viewpoint of making the film larger in area, flexible, and lighter in weight and reducing the cost. Further, patent document 2 proposes a method for forming crystals of pentacene in a dilute o-dichlorobenzene solution without using the vacuum vapor deposition process. However, the method is difficult to perform and it has not yet furnished a stable device. Another problem with acene type polycyclic aromatic hydrocarbon molecules such as pentacene is poor oxidation stability.
Studies on organic thin film solar cells had initially been conducted by the use of single-layer films made from merocyanine dyes and the like. Meanwhile, a multilayer film consisting of a p-layer that transports holes and an n-layer that transports electrons was found to improve the conversion efficiency of optical input to electric output (photovoltaic conversion efficiency) and thereafter the multilayer film design has become the mainstream. In the early days of studies on multilayer films, copper phthalocyanine (CuPc) was used for the p-layer and a peryleneimide (for example, PTCBI) for the n-layer. On the other hand, the studies on organic thin film solar cells using polymers were primarily focused on the so-called bulk heterojunction wherein an electrically conductive polymer used as a material for the p-layer and a fullerene (C60) derivative used as a material for the n-layer are mixed together and thermally treated to induce microphase separation thereby increasing the heterointerface and enhancing the photovoltaic conversion efficiency. The materials mainly used in these studies were poly(3-hexylthiophene) (P3HT) for the p-layer and C60 derivative (DCBM) for the n-layer.
As described above, not much progress has been achieved in the materials for both layers of organic thin film solar cells and phthalocyanine derivatives, peryleneimide derivatives, and C60 derivatives are still used today. Therefore, there has been a strong demand for development of novel materials to replace the conventional materials in order to enhance the photovoltaic conversion efficiency. For example, patent document 3 discloses an organic thin film solar cell in which a compound having a fluoranthene skeleton is used, but this does not yield satisfactory photovoltaic conversion efficiency.
Patent document 4 discloses the indoloindole compound illustrated below. However, the document merely discloses indoloindole compounds having a skeleton formed by [3,2-b] fusion and organic transistors using these compounds.

Patent documents 5 and 6 disclose organic EL devices using the compounds illustrated below. However, the documents merely disclose compounds having a benzochalcogeno[3,2-b]benzochalcogenophene skeleton and organic EL devices using these compounds.
